(a) Field of the Invention
This invention relates generally to the field of Aeroacoustical Wind Tunnel Testing for jet engine nozzle assembly noise level evaluation and in particular to a tone injected nacelle for such testing.
(b) Background of the Invention
Excess jet noise is a phenomenon in which a jet engine internal noise, such as a fan tone, can excite the exhaust jet to produce a significant increase in the broadband jet noise spectrum. This effect must be taken into consideration to establish accurate jet noise prediction procedures, which have generally in the past been based on noise measurements of "clean" laboratory jets, and, consequently, have tended to underpredict real jet engine noise.
Applicant's assignee has heretofore developed an aeroacoustical test apparatus for making such predictions which positions a plurality of high frequency acoustical drivers around the primary and secondary flow ducts of a jet engine to simulate internal noise sources. A typical high by-pass ratio jet engine is known to have internal noise radiation out of both the core (primary) and fan (secondary) nozzles. It has been found that while acoustical excitations in the primary duct produced modest excess jet noise, acoustical excitation in the secondary duct can generate significant excess jet noise and hence acoustical excitation in the secondary duct is most effective in jet noise amplification so that acoustical drivers have been used to direct noise laterally into the secondary duct Previously, the acoustical drivers have been positioned radially around the secondary duct in order to direct the noise laterally, into the secondary or bypass duct This approach has a number of disadvantages in that the positioning of acoustical drivers at the primary and secondary flow ducts have yielded a large configuration with poor aerodynamics which could only be used in the static environment of a large test chamber so that further testing was needed in wind tunnels to determine effects of excess jet noise.